Motor having concentratedly-wound stator coil

ABSTRACT

A motor includes a stator that has pole cores and a stator coil comprised of unit coils each being concentratedly wound around a corresponding one of the pole cores. Each of the unit coils is comprised of first and second sub-coils that are stacked in two layers in a stacking direction. Each of the first and second sub-coils is spirally wound so that coil sides of the sub-coil overlap each other in an overlapping direction perpendicular to the stacking direction. Each of the unit coils also has, at a single place, a connecting portion that connects the first and second sub-coils of the unit coil. The connecting portion is provided in a coil end of the unit coil. For each of the first and second sub-coils of the unit coil, the connecting portion is located at an innermost periphery of the sub-coil in the overlapping direction of the sub-coil.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority from Japanese PatentApplication No. 2015-178772 filed on Sep. 10, 2015, the content of whichis hereby incorporated by reference in its entirety into thisapplication.

BACKGROUND

1 Technical Field

The present invention relates to motors having a stator coil comprisedof a plurality of unit coils, each of the unit coils beingconcentratedly wound around one of a plurality of pole cores in twolayers in a height direction of the pole core.

2 Description of Related Art

To achieve both size reduction and output increase of a motor, there isavailable a method of regularly winding a stator coil of the motor andthereby improving the space factors of the stator coil in inter-polecore spaces (i.e., spaces between circumferentially adjacent pole coresof the stator of the motor). However, in the case of the stator coilbeing a concentratedly-wound coil, lead portions of the stator coil maybecome obstacles to the process of regularly winding the stator coil.

To solve the above problem, Japanese Patent Application Publication No.JP2012175741A discloses a concentratedly-wound stator coil. The statorcoil includes a plurality of parts each being formed by spirally windinga rectangular wire. The spirally-wound parts are stacked in a pluralityof layers. Moreover, those portions of the spirally-wound parts whichconstitute coil ends of the stator coil are bent into an arc shape.Furthermore, the stator coil also includes a plurality of connectingportions (or layer-changing portions) each connecting one adjacent pairof the spirally-wound parts of the stator coil. The connecting portionsare arranged in the inter-pole core spaces and thus not bent into thearc shape.

With the above configuration, it is possible to regularly andconcentratedly wind the stator coil. However, at the same time, thespace factors of the stator coil in the inter-pole core spaces arelowered due to the arrangement of the connecting portions of the statorcoil in the inter-pole core spaces.

SUMMARY

According to exemplary embodiments, there is provided a motor whichincludes a rotor and a stator that is opposed to the rotor with a radialgap or an axial gap formed therebetween. The stator includes a pluralityof pole cores and a stator coil comprised of a plurality of unit coilseach of which is concentratedly wound around a corresponding one of thepole cores. Each of the unit coils is comprised of a pair of first andsecond sub-coils that are stacked in two layers in a stacking direction;the stacking direction coincides with a height direction of thecorresponding pole core. Each of the first and second sub-coils isspirally wound so that coil sides of the sub-coil overlap each other inan overlapping direction perpendicular to the stacking direction. Eachof the unit coils also has, at a single place, a connecting portion thatconnects the first and second sub-coils of the unit coil. The connectingportion is provided in a coil end of the unit coil. For each of thefirst and second sub-coils of the unit coil, the connecting portion islocated at an innermost periphery of the sub-coil in the overlappingdirection of the sub-coil.

With the above configuration, since the connecting portion is located atthe innermost peripheries of the first and second sub-coils, it becomespossible to provide lead portions of the unit coil at the outermostperipheries of the first and second sub-coils. Consequently, it becomespossible for the lead portions to extend without intersecting other coilsides of the first and second sub-coils. As a result, it becomespossible to regularly wind each of the unit coils around thecorresponding one of the pole cores.

Moreover, since the connecting portions of the unit coils are providedin the coil ends of the unit coils, the connecting portions cannotbecome obstacles to the process of regularly winding the unit coils (orthe stator coil) in the inter-pole core spaces. Consequently, it becomespossible to maximize the space factors of the unit coils in theinter-pole core spaces, thereby achieving both size reduction and outputincrease of the motor.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription given hereinafter and from the accompanying drawings ofexemplary embodiments, which, however, should not be taken to limit theinvention to the specific embodiments but are for the purpose ofexplanation and understanding only.

In the accompanying drawings:

FIG. 1 is a cross-sectional view of a motor according to a firstembodiment, which is taken along the line I-I in FIG. 2 and from whichhatching lines are omitted for the sake of simplicity;

FIG. 2 is a partially cross-sectional view of an engine starter whichincludes the motor according to the first embodiment;

FIG. 3 is a plan view of a pair of first and second sub-coils, the firstand second sub-coils together constituting one of four unit coils of astator coil of the motor according to the first embodiment;

FIG. 4 is a perspective view of a stator coil assembly according to thefirst embodiment;

FIG. 5 is a perspective view of a stator assembly according to the firstembodiment;

FIG. 6 is a cross-sectional view, taken perpendicular to an axialdirection, of a motor according to a second embodiment;

FIG. 7 is a plan view of a pair of first and second sub-coils, the firstand second sub-coils together constituting one of four unit coils of astator coil of the motor according to the second embodiment;

FIG. 8 is a perspective view of a stator coil assembly according to athird embodiment;

FIG. 9 is a cross-sectional view, taken perpendicular to an axialdirection, of a motor according to a fourth embodiment;

FIG. 10 is a plan view of a pair of first and second sub-coils, thefirst and second sub-coils together constituting one of four unit coilsof a stator coil of the motor according to the fourth embodiment;

FIG. 11 is a perspective view of a stator coil assembly according to thefourth embodiment;

FIG. 12 is a cross-sectional view, taken perpendicular to an axialdirection, of a motor according to a fifth embodiment;

FIG. 13 is a cross-sectional view, taken perpendicular to an axialdirection, of a motor according to a sixth embodiment; and

FIG. 14 is a cross-sectional view, taken perpendicular to an axialdirection, of a motor according to a seventh embodiment.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments will be described hereinafter with reference toFIGS. 1-14. It should be noted that for the sake of clarity andunderstanding, identical components having identical functionsthroughout the whole description have been marked, where possible, withthe same reference numerals in each of the figures and that for the sakeof avoiding redundancy, descriptions of the identical components willnot be repeated.

First Embodiment

FIG. 1 shows the overall configuration of a motor 2 according to a firstembodiment.

In the present embodiment, the motor 2 is designed to be used in astarter 1 for starting an internal combustion engine of a motor vehicle.

Referring to FIG. 2, the starter 1 is configured to shift a pinion 4arranged on an output shaft 3, utilizing an attraction force created byan electromagnetic switch 5, together with a clutch 6 in a directionaway from the motor 2 (i.e., the leftward direction in FIG. 2), therebybringing the pinion 4 into mesh with a ring gear (not shown) of theengine.

The configuration and operation of the starter 1 are similar to those ofengine starters well-known in the art. Therefore, a detailed descriptionof the configuration and operation of the starter 1 will be omittedhereinafter.

As shown in FIG. 1, the motor 2 includes a stator 7 that constitutes anelectromagnet-type field and a rotor 8 that constitutes an armature. Inthe present embodiment, the motor 2 is configured as a radial gap-typeDC (Direct Current) motor such that the rotor 8 is rotatably disposedradially inside the stator 7 with a radial gap formed therebetween.

The rotor 8 includes an armature shaft (or rotor shaft) 9, an armaturecore (or rotor core) 10 fixedly fitted on an outer periphery of thearmature shaft 9, an armature coil 11 mounted on the armature core 10,and a commutator 12 provided on a rear end portion of the armature shaft9 (see FIG. 2).

The commutator 12 includes a base member 12 a and a plurality ofcommutator segments 12 b. The base member 12 a is formed of anelectrically-insulative resin material into a hollow cylindrical shape.The base member 12 a is fixedly fitted on an outer periphery of the rearend portion of the armature shaft 9. The commutator segments 12 b arearranged on a radially outer surface of the base member 12 a along acircumferential direction thereof so as to be spaced from one another atequal intervals. Consequently, the commutator segments 12 b areelectrically insulated from one another by the base member 12 a.Moreover, the commutator segments 12 b are electrically connected to thearmature coil 11 via risers provided on the side of the armature core10.

On a radially outside of the commutator 12, there are arranged fourbrushes 13 so as to be circumferentially spaced from one another atequal intervals. The brushes 13 are pressed by respective brush springs14 against the outer surfaces of the commutator segments 12 b. Inaddition, in the present embodiment, each of the brush springs 14 isimplemented by a plate spring as shown in FIG. 2. However, it should benoted that each of the brush springs 14 may be alternatively implementedby a helical spring.

Referring back to FIG. 1, the stator 7 includes a hollow cylindricalyoke 15 for forming a magnetic circuit, a plurality (e.g., four in thepresent embodiment) of pole cores 16 fixed to a radially inner peripheryof the yoke 15, and a stator coil that generates, upon being suppliedwith electric current, magnetic field and thereby magnetizes the polecores 16. In addition, the yoke 15 also serves as an outer shell of themotor 2.

The pole cores 16 are arranged in a circumferential direction of theyoke 15 at equal intervals and each extend radially inward from the yoke15. Each of the pole cores 16 is fixed to the radially inner peripheryof the yoke 15 by a screw 17. Hereinafter, for each of the pole cores16, the radial direction in which the pole core 16 extends inward fromthe yoke 15 and faces another one of the pole cores 16 will be referredto as the height direction of the pole core 16.

Each of the pole cores 16 has a pair of collar portions 16 a formed at adistal end (i.e., the end on the opposite side to the yoke 15 in theheight direction of the pole core 16) thereof so as to extendrespectively toward opposite sides in the circumferential direction ofthe yoke 15. Each of the collar portions 16 a also extends in the axialdirection of the yoke 15 over the entire axial length of the pole core16. Moreover, each of the collar portions 16 a has an arc-shaped crosssection perpendicular to the axial direction of the yoke 15 (or theaxial direction of the motor 2) and radially faces the armature core 10with a small radial gap formed therebetween.

The stator coil is comprised of a plurality (e.g., four in the presentembodiment) of unit coils 18 each of which is concentratedly andregularly wound around a corresponding one of the pole cores 16.

Referring further to FIGS. 3-5 together with FIG. 1, each of the unitcoils 18 is comprised of a pair of first and second sub-coils 20 and 21that are stacked in two layers in the height direction of thecorresponding pole core 16. The first sub-coil 20 is located at thefirst layer (i.e., the layer on the yoke 15 side), while the secondsub-coil 21 is located at the second layer (i.e., the layer on the rotor8 side).

As shown in FIG. 3, each of the first and second sub-coils 20 and 21 isformed by spirally winding an electrically-conductive wire having asubstantially square cross-sectional shape around a winding axis, sothat coil sides of the sub-coil overlap each other in a directionperpendicular to both the winding axial direction (i.e., a directionperpendicular to the plane of FIG. 3) and the extending direction (orthe longitudinal direction) of the sub-coil. Hereinafter, the directionin which coil sides of the sub-coil overlap each other will be referredto as the overlapping direction of the sub-coil.

Moreover, the first and second sub-coils 20 and 21 are stacked so thatboth the winding axial directions of the first and second sub-coils 20and 21 coincide with the height direction of the corresponding pole core16. That is, the stacking direction of the first and second sub-coils 20and 21 coincides with the height direction of the corresponding polecore 16.

Furthermore, the first and second sub-coils 20 and 21 are connected witheach other at a single place by a connecting portion (or layer-changingportion) 19. As shown in FIG. 3, part of the connecting portion 19 isincluded in the first sub-coil 20, while the remainder of the connectingportion 19 is included in the second sub-coil 21.

In the present embodiment, the connecting portion 19 is provided in oneof two coil ends of the unit coil 18 (more particularly, in therear-side coil end of the unit coil 18). Here, the two coil ends of theunit coil 18 denote those two parts of the unit coil 18 which arelocated respectively on opposite axial sides of the corresponding polecore 16 and thus outside the inter-pole core spaces (i.e. the spacesbetween circumferentially adjacent pole cores 16) of the stator 7.Further, for each of the first and second sub-coils 20 and 21, theconnecting portion 19 is located at the innermost periphery of thesub-coil in the overlapping direction of the sub-coil.

Furthermore, the connecting portion 19 is plastically deformed, bypressing the wire in the overlapping direction, to have its dimension inthe overlapping direction (to be referred to as thickness hereinafter)reduced and its dimension in the stacking direction (to be referred toas width hereinafter) increased. More particularly, in the presentembodiment, the thickness of the connecting portion 19 in theoverlapping direction is substantially ½ of the thickness of the otherportions of the unit coil 18 in the overlapping direction; and the widthof the connecting portion 19 in the stacking direction is substantiallytwice the width of the other portions of the unit coil 18 in thestacking direction.

Moreover, the number of turns of the first sub-coil 20 is set to beequal to the number of turns of the second sub-coil 21. Moreparticularly, in the present embodiment, both the number of turns of thefirst sub-coil 20 and the number of turns of the second sub-coil 21 areset to 3. Consequently, the number of coil sides of the first sub-coil20 and the number of coil sides of the second sub-coil 21 received inthe same inter-pole core space are equal to each other.

Each of winding finish ends of the first and second sub-coils 20 and 21is led out, as a terminal portion 18 a for electrical connection of theunit coil 18 with the other unit coils 18, to the rear side (i.e., theupper side in FIG. 3). In addition, hereinafter, the outermost coil sideof the first sub-coil 20 connected with the terminal portion 18 a of thefirst sub-coil 20 will be referred to as a first lead portion 20 a; theoutermost coil side of the second sub-coil 21 connected with theterminal portion 18 a of the second sub-coil 21 will be referred to as asecond lead portion 21 a.

Each of the unit coils 18 is wound around the corresponding pole core 16so that the first and second lead portions 20 a and 21 a of the unitcoil 18 are respectively located on opposite circumferential sides ofthe corresponding pole core 16.

Moreover, as shown in FIG. 4, each of the unit coils 18 has its firstlead portion 20 a arranged to circumferentially adjoin the first leadportion 20 a of another unit coil 18 and its second lead portion 21 aarranged to circumferentially adjoin the second lead portion 21 a of yetanother unit coil 18. In other words, for eachcircumferentially-adjacent pair of the unit coils 18, either the firstlead portions 20 a of the pair of the unit coils 18 or the second leadportions 21 a of the pair of the unit coils 18 are arranged tocircumferentially adjoin each other.

Furthermore, all of the four unit coils 18 are assembled together toform a stator coil assembly as shown in FIG. 4. In the stator coilassembly, two pairs of circumferentially adjoining terminal portions 18a of the first sub-coils 20 of the unit coils 18 are joined to aconnection bar 22; one pair of circumferentially adjoining terminalportions 18 a of the second sub-coils 21 of the unit coils 18 is joinedto a pigtail 23 of one positive-side brush 13; and another pair ofcircumferentially adjoining terminal portions 18 a of the secondsub-coils 21 of the unit coils 18 is joined to a pigtail 23 of anotherpositive-side brush 13.

Further, a motor lead wire 25 is joined to the connection bar 22; themotor lead wire 25 is later to be electrically connected to a motor-sideterminal bolt 24 of the electromagnetic switch 5 of the starter 1 (seeFIG. 2). Then, the four pole cores 16 are respectively inserted into thefour unit coils 18 from the radially inside. Thereafter, the four polecores 16 are fixed to the radially inner periphery of the yoke 15respectively by the four screws 17. Consequently, a stator assembly asshown in FIG. 5 is obtained.

According to the present embodiment, it is possible to achieve thefollowing advantageous effects.

In the present embodiment, the motor 2 includes the rotor 8 and thestator 7 that is opposed to the rotor 8 with the radial gap formedtherebetween. The stator 7 includes the four pole cores 16 and thestator coil comprised of the four unit coils 18 each of which isconcentratedly wound around the corresponding one of the pole cores 16.Each of the unit coils 18 is comprised of the pair of first and secondsub-coils 20 and 21 that are stacked in two layers in the stackingdirection; the stacking direction coincides with the height direction ofthe corresponding pole core 16. Each of the first and second sub-coils20 and 21 is spirally wound so that coil sides of the sub-coil overlapeach other in the overlapping direction perpendicular to the stackingdirection. Each of the unit coils 18 also has, at a single place, theconnecting portion 19 that connects the first and second sub-coils 20and 21 of the unit coil 18. The connecting portion 19 is provided in therear-side coil end of the unit coil 18. For each of the first and secondsub-coils 20 and 21 of the unit coil 18, the connecting portion 19 islocated at the innermost periphery of the sub-coil in the overlappingdirection of the sub-coil.

With the above configuration, since the connecting portion 19 is locatedat the innermost peripheries of the first and second sub-coils 20 and21, it becomes possible to provide the first and second lead portions 20a and 21 a respectively at the outermost peripheries of the first andsecond sub-coils 20 and 21. Consequently, it becomes possible for thefirst and second lead portions 20 a and 21 a to extend withoutintersecting other coil sides of the first and second sub-coils 20 and21. As a result, it becomes possible to regularly wind each of the unitcoils 18 around the corresponding one of the pole cores 16.

Moreover, since the connecting portions 19 of the unit coils 18 areprovided in the rear-side coil ends of the unit coils 18, the connectingportions 19 cannot become obstacles to the process of regularly windingthe unit coils 18 (or the stator coil) in the inter-pole core spaces.Consequently, it becomes possible to maximize the space factors of theunit coils 18 in the inter-pole core spaces, thereby achieving both sizereduction and output increase of the motor 2.

In the present embodiment, for each of the unit coils 18, the connectingportion 19 of the unit coil 18 has its thickness set to be less than thethickness of other portions of the unit coil 18 in the overlappingdirection.

Setting the thickness of the connecting portion 19 as above, it ispossible to reduce the axial length of the unit coil 18 and thus theaxial length of the entire motor 2, thereby achieving reduction in thesize and weight of the motor 2.

Further, in the present embodiment, for each of the unit coils 18, theconnecting portion 19 of the unit coil 18 has its thickness set to besubstantially ½ of the thickness of the other portions of the unit coil18 in the overlapping direction and its width set to be substantiallytwice the width of the other portions of the unit coil 18 in thestacking direction.

Setting the thickness and width of the connecting portion 19 as above,it becomes possible to keep the cross-sectional area of the connectingportion 19 substantially equal to those of the other portions of theunit coil 18. Consequently, it becomes possible to secure highresistance of the unit coil 18 to heat that is generated upon supply ofelectric current to the unit coil 18.

In the present embodiment, each of the unit coils 18 is configured tohave the quadrangular (more particularly, square) cross-sectional shape.

Consequently, compared to the case of configuring each of the unit coils18 to have a circular cross-sectional shape, it becomes possible toimprove the space factors of the unit coils 18 in the inter-pole corespaces.

In the present embodiment, for each circumferentially-adjacent pair ofthe unit coils 18, either the first lead portions 20 a of the pair ofthe unit coils 18 or the second lead portions 21 a of the pair of theunit coils 18 are arranged to adjoin each other in the circumferentialdirection of the stator 7 (or in the overlapping direction). Moreover,each circumferentially-adjoining pair of the first lead portions 20 a orthe second lead portions 21 a are electrically connected with each othervia the corresponding terminal portions 18 a of the pair of the unitcoils 18.

With the above arrangement of the first and second lead portions 20 aand 21 a of the unit coils 18, it becomes possible to minimize theelectrical connection path between each circumferentially-adjacent pairof the unit coils 18.

In the present embodiment, the motor 2 is used in the starter 1 forstarting an internal combustion engine.

In general, a starter motor is operated at low voltage and high electriccurrent. Therefore, the motor 2 according to the present embodiment,which has a small number of turns per pole and high space factors, isparticularly suitable for use in the starter 1.

Second Embodiment

In this embodiment, as shown in FIG. 6, for eachcircumferentially-adjacent pair of the unit coils 18, both the unitcoils 18 of the pair are partially received in a same one of theinter-pole core spaces. In the same inter-pole core space, the number ofoverlapping coil sides (or the number of turns) of the first sub-coil 20and the number of overlapping coil sides of the second sub-coil 21 setfor one of the two unit coils 18 of the pair are respectively differentfrom those set for the other of the two unit coils 18.

More specifically, in the present embodiment, as shown in FIG. 7, foreach of the unit coils 18, the number of overlapping coil sides of thefirst sub-coil 20 of the unit coil 18 is set to be equal to four on theleft side of a centerline of the first sub-coil 20 and equal to three onthe right side of the centerline. That is, the number of overlappingcoil sides of the first sub-coil 20 on the left side of the centerlineof the first sub-coil 20 is set to be greater than that on the rightside of the centerline by one. Moreover, the outermost coil side of thefirst sub-coil 20 on the left side of the centerline constitutes thefirst lead portion 20 a of the unit coil 18. On the other hand, thenumber of overlapping coil sides of the second sub-coil 21 of the unitcoil 18 is set to be equal to three on the left side of a centerline ofthe second sub-coil 21 and equal to four on the right side of thecenterline. That is, the number of overlapping coil sides of the secondsub-coil 21 on the left side of the centerline of the second sub-coil 21is set to be less than that on the right side of the centerline by one.Moreover, the outermost coil side of the second sub-coil 21 on the rightside of the centerline constitutes the second lead portion 21 a of theunit coil 18. In addition, in FIG. 7, the centerlines of the first andsecond sub-coils 20 and 21 are shown with one-dot chain lines.

With the above configuration of the unit coils 18, referring back toFIG. 6, for each circumferentially-adjacent pair of the unit coils 18,in the inter-pole core space where both the unit coils 18 of the pairare partially received, one of the two unit coils 18 of the pair has thenumber of overlapping coil sides of the first sub-coil 20 set to begreater than the number of overlapping coil sides of the second sub-coil21 by one; and the other one of the two unit coils 18 of the pair hasthe number of overlapping coil sides of the second sub-coil 21 set to begreater than the number of overlapping coil sides of the first sub-coil20 by one.

Moreover, in the present embodiment, as shown in FIG. 6, for eachcircumferentially-adjacent pair of the unit coils 18, the first leadportion 20 a of one of the two unit coils 18 of the pair and the secondlead portion 21 a of the other of the two unit coils 18 of the pair arearranged to radially overlap each other (or arranged at the samecircumferential position).

With the above arrangement of the first and second lead portions 20 aand 21 a of the unit coils 18, it becomes possible to regularly wind theunit coils 18, which have the configuration as shown in FIG. 7, in theinter-pole core spaces without leaving unnecessary gaps therein.Consequently, it becomes possible to secure high space factors of theunit coils 18 in the inter-pole core spaces, thereby achieving both sizereduction and output increase of the motor 2.

Third Embodiment

This embodiment illustrates a method of electrically connecting thefirst and second lead portions 20 a and 21 a of eachcircumferentially-adjacent pair of the unit coils 18 described in thesecond embodiment.

As described previously, in the first embodiment, for eachcircumferentially-adjacent pair of the unit coils 18, either the firstlead portions 20 a of the pair of the unit coils 18 or the second leadportions 21 a of the pair of the unit coils 18 are arranged tocircumferentially adjoin each other (see FIG. 4). In comparison, in thesecond embodiment, for each circumferentially-adjacent pair of the unitcoils 18, the first lead portion 20 a of one of the two unit coils 18 ofthe pair and the second lead portion 21 a of the other of the two unitcoils 18 of the pair are arranged to radially overlap each other (seeFIG. 6).

In the present embodiment, the first and second lead portions 20 a and21 a of each circumferentially-adjacent pair of the unit coils 18described in the second embodiment are electrically connected asfollows.

As shown in FIG. 8, for each circumferentially-adjacent pair of the unitcoils 18, the terminal portion 18 a of the first sub-coil 20 of one ofthe two unit coils 18 of the pair is bent, against the first leadportion 20 a of the one of the two unit coils 18, radially inward andarranged to circumferentially adjoin the terminal portion 18 a of thesecond sub-coil 21 of the other of the two unit coils 18. Consequently,there are a total of four pairs of circumferentially adjoining terminalportions 18 a of the unit coils 18. Among them, two pairs ofcircumferentially adjoining terminal portions 18 a of the unit coils 18are joined to a connection bar 22; one pair of circumferentiallyadjoining terminal portions 18 a of the unit coils 18 is joined to apigtail 23 of one positive-side brush 13; and the remaining one pair ofcircumferentially adjoining terminal portions 18 a of the unit coils 18is joined to a pigtail 23 of another positive-side brush 13. Further, amotor lead wire 25 is joined to the connection bar 22; the motor leadwire 25 is later to be electrically connected to the motor-side terminalbolt 24 of the electromagnetic switch 5 of the starter 1 (see FIG. 2).

With the above method, it becomes possible to minimize the electricalconnection path between each circumferentially-adjacent pair of the unitcoils 18 described in the second embodiment.

In an alternative method, for each circumferentially-adjacent pair ofthe unit coils 18, the terminal portion 18 a of the first sub-coil 20 ofone of the two unit coils 18 of the pair may be led out straight,without being bent radially inward, from the first lead portion 20 a ofthe one of the two unit coils 18 and arranged to radially adjoin theterminal portion 18 a of the second sub-coil 21 of the other of the twounit coils 18.

Fourth Embodiment

In this embodiment, each of the first and second lead portions 20 a and21 a of the unit coils 18 is reduced in the thickness in the overlappingdirection and increased in the width in the stacking direction.

Specifically, in the present embodiment, as shown in FIG. 10, for eachof the unit coils 18, the number of overlapping coil sides (or thenumber of turns) of the first sub-coil 20 of the unit coil 18 is set tobe equal to four on the left side of a centerline of the first sub-coil20 and equal to three on the right side of the centerline. That is, thenumber of overlapping coil sides of the first sub-coil 20 on the leftside of the centerline of the first sub-coil 20 is set to be greaterthan that on the right side of the centerline by one. Moreover, theoutermost coil side of the first sub-coil 20 on the left side of thecenterline constitutes the first lead portion 20 a of the unit coil 18.On the other hand, the number of overlapping coil sides of the secondsub-coil 21 of the unit coil 18 is set to be equal to three on the leftside of a centerline of the second sub-coil 21 and equal to four on theright side of the centerline. That is, the number of overlapping coilsides of the second sub-coil 21 on the left side of the centerline ofthe second sub-coil 21 is set to be less than that on the right side ofthe centerline by one. Moreover, the outermost coil side of the secondsub-coil 21 on the right side of the centerline constitutes the secondlead portion 21 a of the unit coil 18. In addition, in FIG. 10, thecenterlines of the first and second sub-coils 20 and 21 are shown withone-dot chain lines.

Moreover, in the present embodiment, each of the first and second leadportions 20 a and 21 a of the unit coils 18 is plastically deformed, bypressing the wire in the overlapping direction, to have its thickness inthe overlapping direction reduced and its width in the stackingdirection increased.

More particularly, in the present embodiment, as shown in FIG. 9, thethickness of the first and second lead portions 20 a and 21 a in theoverlapping direction is ½ of the thickness of the other coil sides ofthe unit coils 18 in the overlapping direction; and the width of thefirst and second lead portions 20 a and 21 a in the stacking directionis twice the width of the other coil sides of the unit coils 18 in thestacking direction.

Furthermore, in the present embodiment, each of the first and secondlead portions 20 a and 21 a of the unit coils 18 is arranged over boththe first and second layers of the unit coils 18 in the stackingdirection. Moreover, each of the first lead portions 20 a of the unitcoils 18 is arranged to circumferentially adjoin a corresponding one ofthe second lead portions 21 a of the unit coils 18.

Moreover, in the present embodiment, as shown in FIG. 9, each of theunit coils 18 has the number of overlapping coil sides of the firstsub-coil 20 set to be greater than the number of overlapping coil sidesof the second sub-coil 21 by one in one of the two inter-pole corespaces between which the corresponding pole core 16 is interposed andthe number of overlapping coil sides of the second sub-coil 21 set to begreater than the number of overlapping coil sides of the first sub-coil20 by one in the other of the two inter-pole core spaces.

In the present embodiment, as shown in FIG. 11, for eachcircumferentially-adjoining pair of the first and second lead portions20 a and 21 a of the unit coils 18, the pair of the terminal portions 18a of the unit coils 18 respectively connected with the pair of the firstand second lead portions 20 a and 21 a are also arranged tocircumferentially adjoin each other. Consequently, there are a total offour pairs of circumferentially adjoining terminal portions 18 a of theunit coils 18. Among them, two pairs of circumferentially adjoiningterminal portions 18 a of the unit coils 18 are joined to a connectionbar 22; one pair of circumferentially adjoining terminal portions 18 aof the unit coils 18 is joined to a pigtail 23 of one positive-sidebrush 13; and the remaining one pair of circumferentially adjoiningterminal portions 18 a of the unit coils 18 is joined to a pigtail 23 ofanother positive-side brush 13. Further, a motor lead wire 25 is joinedto the connection bar 22; the motor lead wire 25 is later to beelectrically connected to the motor-side terminal bolt 24 of theelectromagnetic switch 5 of the starter 1 (see FIG. 2).

With the above arrangement of the first and second lead portions 20 aand 21 a of the unit coils 18 according to the present embodiment, itbecomes possible to regularly wind the unit coils 18, which have theconfiguration as shown in FIG. 10, in the inter-pole core spaces withoutleaving unnecessary gaps therein. Consequently, it becomes possible tosecure high space factors of the unit coils 18 in the inter-pole corespaces, thereby achieving both size reduction and output increase of themotor 2.

Fifth Embodiment

In this embodiment, as shown in FIG. 12, each of the pole cores 16 hasan opposite pair of circumferential side surfaces; each of thecircumferential side surfaces is stepped in the height direction of thepole core 16 at a position corresponding to the boundary between thefirst and second sub-coils 20 and 21 of the corresponding unit coil 18in the stacking direction. Consequently, a recess 16 b is formed in thecircumferential side surface so as to be circumferentially recessedtoward the center of the pole core 16. The recess 16 b is positioned inthe height direction of the pole core 16 on the side of the firstsub-coil 20 of the corresponding unit coil 18.

Moreover, as shown in FIG. 12, for each of the unit coils 18, the numberof turns of the first sub-coil 20 of the unit coil 18 is set to begreater than the number of turns of the second sub-coil 21 of the unitcoil 18. More particularly, in the present embodiment, the number ofturns of the first sub-coil 20 is set to four, while the number of turnsof the second sub-coil 21 is set to three.

Furthermore, in the present embodiment, as shown in FIG. 12, in each ofthe recesses 16 b formed in the circumferential side surfaces of thepole cores 16, there is received at least part of an innermost coil sideof the first sub-coil 20 of the corresponding unit coil 18.

With the above arrangement, it becomes possible to absorb the differencein the number of turns (or the difference in outer diameter) between thefirst and second sub-coils 20 and 21 of the unit coils 18 by the steppedshape of the circumferential side surfaces of the pole cores 16. Thus,no step is formed in the stacking direction between the outermost coilsides of the first and second sub-coils 20 and 21. Consequently, itbecomes possible to regularly wind the unit coils 18 in the inter-polecore spaces without leaving unnecessary gaps therein. As a result, itbecomes possible to secure high space factors of the unit coils 18 inthe inter-pole core spaces, thereby achieving both size reduction andoutput increase of the motor 2.

Sixth Embodiment

In this embodiment, as shown in FIG. 13, each of the pole cores 16 hasan opposite pair of circumferential side surfaces; each of thecircumferential side surfaces is stepped in the height direction of thepole core 16 at a position corresponding to the boundary between thefirst and second sub-coils 20 and 21 of the corresponding unit coil 18in the stacking direction. Consequently, a recess 16 b is formed in thecircumferential side surface so as to be circumferentially recessedtoward the center of the pole core 16. The recess 16 b is radiallypositioned on the side of the second sub-coil 21 of the correspondingunit coil 18 (or on the radially inner side).

Moreover, as shown in FIG. 13, for each of the unit coils 18, the numberof turns of the first sub-coil 20 of the unit coil 18 is set to be equalto the number of turns of the second sub-coil 21 of the unit coil 18.

Furthermore, in the present embodiment, as shown in FIG. 13, in each ofthe recesses 16 b formed in the circumferential side surfaces of thepole cores 16, there is received at least part of an innermost coil sideof the second sub-coil 21 of the corresponding unit coil 18.

In the stator 7 of the radial gap-type motor 2, when the number of turnsof the first sub-coil 20 is set to be equal to the number of turns ofthe second sub-coil 21 as in the present embodiment, the annular rangeof the second sub-coil 21 located on the radially inner side is widerthan the angular range of the first sub-coil 20 located on the radiallyouter side.

However, with the above configuration of the stator 7 according to thepresent embodiment, it becomes possible to absorb the difference inangular range between the first and second sub-coils 20 and 21 of theunit coils 18 by the stepped shape of the circumferential side surfacesof the pole cores 16. Consequently, it becomes possible to regularlywind the unit coils 18 in the inter-pole core spaces without leavingunnecessary gaps therein. As a result, it becomes possible to securehigh space factors of the unit coils 18 in the inter-pole core spaces,thereby achieving both size reduction and output increase of the motor2.

Seventh Embodiment

In the present embodiment, as shown in FIG. 14, for each of the unitcoils 18, the number of turns of the first sub-coil 20 of the unit coil18 is set to be greater than the number of turns of the second sub-coil21 of the unit coil 18. More specifically, the number of turns of thefirst sub-coil 20 is set to four, while the number of turns of thesecond sub-coil 21 is set to three.

Moreover, each circumferentially-adjacent pair of the unit coils 18 arearranged with an inter-coil gap formed between outermost coil sides ofthe second sub-coils 21 of the pair of the unit coils 18. In addition,between each circumferentially-facing pair of the collar portions 16 aof the pole cores 16, there is formed an inter-collar portion gap.

The stator 7 further includes a plurality (e.g., four in the presentembodiment) of magnets 26 each of which is arranged in both acorresponding one of the inter-collar portion gaps and a correspondingone of the inter-coil gaps. That is, between eachcircumferentially-adjacent pair of the pole cores 16, there is arrangeda corresponding one of the magnets 26.

Moreover, each of the magnets 26 is configured (or magnetized) togenerate magnetic flux in an opposite direction to magnetic flux that isgenerated by the corresponding unit coils 18 and flows between thecorresponding pair of the collar portions 16 a of the pole cores 16through the corresponding inter-collar portion gap formed therebetween.In addition, in FIG. 14, the direction of the magnetic flux generated byone of the magnets 26 is indicated by an arrowed dashed-line B; and thedirection of the magnetic flux generated by the corresponding unit coils18 is indicated by an arrowed dashed-line A.

With the above configuration, it becomes possible to reduce leakagemagnetic flux between each circumferentially-facing pair of the collarportions 16 a of the pole cores 16. That is, it becomes possible toeffectively utilize the inter-coil gaps formed between circumferentiallyadjacent unit coils 18. Consequently, it becomes possible to achieveboth size reduction and output increase of the motor 2.

While the above particular embodiments have been shown and described, itwill be understood by those skilled in the art that the presentinvention can also be embodied in various other modes without departingfrom the spirit of the present invention.

For example, in the first embodiment, each of the first and secondsub-coils 20 and 21 of the unit coils 18 is formed by winding anelectrically-conductive wire that has a substantially square crosssection.

However, each of the first and second sub-coils 20 and 21 of the unitcoils 18 may also be formed by edge-wise-winding anelectrically-conductive wire that has a substantially rectangular crosssection. Here, the term edge-wise-winding denotes the manner of windingthe electrically-conductive wire with the longer sides of therectangular cross section of the wire parallel to the overlappingdirection and the shorter sides of the rectangular cross sectionparallel to the stacking direction. In this case, it is possible to formeach of the first and second sub-coils 20 and 21 of the unit coils 18 tohave a smaller number of turns in comparison with the first embodiment.Accordingly, it is also possible to reduce the number of insulatingcoats or insulating members used for electrically insulating theoverlapping coil sides of the first and second sub-coils 20 and 21 ofthe unit coils 18 from each other. Consequently, it is possible tofurther improve the space factors of the unit coils 18 in the inter-polecore spaces.

Alternatively, each of the first and second sub-coils 20 and 21 of theunit coils 18 may also be formed by winding an electrically-conductivewire having a substantially trapezoidal cross section that has a greaterwidth on the radially outer side than on the radially inner side. Inthis case, it is possible to reduce unnecessary gaps between coil sidesadjacent to each other in the overlapping direction, thereby furtherimproving the space factors of the unit coils 18 in the inter-pole corespaces.

In the first to the seventh embodiments, each of the pole cores 16 hasonly one unit coil 18 wound therearound. However, each of the pole cores16 may also have two or more unit coils 18 wound therearound; the two ormore unit coils 18 are arranged in the height direction of the pole core16. In this case, each circumferentially-adjacent pair of the unit coils18 are located at the same position in the height directions of thecorresponding pole cores 16. Moreover, among all of the unit coils 18,those unit coils 18 which are located closest to the rotor 8 may havethe same configuration and arrangement as the unit coils 18 in theseventh embodiment.

The first to the seventh embodiments are directed to the radial gap-typemotor 2 where the rotor 8 is rotatably disposed radially inside thestator 7 with the radial gap fainted therebetween.

However, the first to the seventh embodiments can also be applied to anouter rotor-type motor where a rotor is rotatably disposed radiallyoutside a stator with a radial gap formed between. Moreover, the firstto the fifth and the seventh embodiments can also be applied to an axialgap-type motor where a stator and a rotor are opposed to each other withan axial gap formed therebetween.

What is claimed is:
 1. A motor comprising: a rotor; and a stator opposedto the rotor with a radial gap or an axial gap formed therebetween, thestator including a plurality of pole cores and a stator coil comprisedof a plurality of unit coils each of which is concentratedly woundaround a corresponding one of the pole cores, wherein each of the unitcoils is comprised of a pair of first and second sub-coils that arestacked in two layers in a stacking direction, the stacking directioncoinciding with a height direction of the corresponding pole core, eachof the first and second sub-coils is spirally wound so that coil sidesof the sub-coil overlap each other in an overlapping directionperpendicular to the stacking direction, each of the unit coils alsohas, at a single place, a connecting portion that connects the first andsecond sub-coils of the unit coil, the connecting portion is provided ina coil end of the unit coil, and for each of the first and secondsub-coils of the unit coil, the connecting portion is located at aninnermost periphery of the sub-coil in the overlapping direction of thesub-coil.
 2. The motor as set forth in claim 1, wherein for each of theunit coils, an outermost coil side of the first sub-coil of the unitcoil, which is connected with a winding finish end of the firstsub-coil, constitutes a first lead portion of the unit coil, and anoutermost coil side of the second sub-coil of the unit coil, which isconnected with a winding finish end of the second sub-coil, constitutesa second lead portion of the unit coil, and for eachcircumferentially-adjacent pair of the unit coils, the first leadportion of one of the two unit coils of the pair and the second leadportion of the other of the two unit coils of the pair are arranged toradially overlap each.
 3. The motor as set forth in claim 2, wherein inthe stator, there are a plurality of inter-pole core spaces each ofwhich is formed between one circumferentially adjacent pair of the polecores, for each circumferentially-adjacent pair of the unit coils, boththe unit coils of the pair are partially received in a same one of theinter-pole core spaces, and in the same inter-pole core space, one ofthe two unit coils of the pair has the number of overlapping coil sidesof the first sub-coil set to be greater than the number of overlappingcoil sides of the second sub-coil by one, and the other one of the twounit coils of the pair has the number of overlapping coil sides of thesecond sub-coil set to be greater than the number of overlapping coilsides of the first sub-coil by one.
 4. The motor as set forth in claim1, wherein for each of the unit coils, the number of turns of the firstsub-coil of the unit coil is set to be greater than the number of turnsof the second sub-coil of the unit coil, each of the pole cores has anopposite pair of circumferential side surfaces, each of thecircumferential side surfaces being stepped at a position in the heightdirection of the pole core to have a recess formed therein, the positioncorresponding to a boundary between the first and second sub-coils ofthe corresponding unit coil in the stacking direction, the recess beingcircumferentially recessed toward a center of the pole core andpositioned in the height direction of the pole core on the side of thefirst sub-coil of the corresponding unit coil, and in the recess, thereis received at least part of an innermost coil side of the firstsub-coil of the corresponding unit coil.
 5. The motor as set forth inclaim 1, wherein the motor is of a radial gap type such that the statoris opposed to the rotor with a radial gap formed therebetween, for eachof the unit coils, the first sub-coil of the unit coil is locatedradially outside the second sub-coil of the unit coil, and the number ofturns of the first sub-coil is set to be equal to the number of turns ofthe second sub-coil, each of the pole cores has an opposite pair ofcircumferential side surfaces, each of the circumferential side surfacesbeing stepped at a position in the height direction of the pole core tohave a recess formed therein, the position corresponding to a boundarybetween the first and second sub-coils of the corresponding unit coil inthe stacking direction, the recess being circumferentially recessedtoward a center of the pole core and radially positioned on the side ofthe second sub-coil of the corresponding unit coil, and in the recess,there is received at least part of an innermost coil side of the secondsub-coil of the corresponding unit coil.
 6. The motor as set forth inclaim 1, wherein in the stator, there are a plurality of inter-pole corespaces each of which is formed between one circumferentially adjacentpair of the pole cores, each of the unit coils has the number ofoverlapping coil sides of the first sub-coil set to be greater than thenumber of overlapping coil sides of the second sub-coil by one in one ofthe two inter-pole core spaces between which the corresponding pole coreis interposed and the number of overlapping coil sides of the secondsub-coil set to be greater than the number of overlapping coil sides ofthe first sub-coil by one in the other of the two inter-pole corespaces, for each of the unit coils, an outermost coil side of the firstsub-coil of the unit coil, which is connected with a winding finish endof the first sub-coil, constitutes a first lead portion of the unitcoil, and an outermost coil side of the second sub-coil of the unitcoil, which is connected with a winding finish end of the secondsub-coil, constitutes a second lead portion of the unit coil, and eachof the first and second lead portions has its thickness set to be ½ ofthe thickness of other coil sides of the unit coil in the overlappingdirection and its width set to be twice the width of the other coilsides in the stacking direction.
 7. The motor as set forth in claim 1,wherein for each of the unit coils, the first sub-coil of the unit coilis located in the height direction of the corresponding pole corefurther than the second sub-coil of the unit coil from the rotor, andthe number of turns of the first sub-coil is set to be greater than thenumber of turns of the second sub-coil, each circumferentially-adjacentpair of the unit coils are arranged with an inter-coil gap formedbetween outermost coil sides of the second sub-coils of the pair of theunit coils, each of the pole cores has a pair of collar portions formedat a rotor-side end thereof in the height direction so as to extendrespectively toward opposite circumferential sides, between eachcircumferentially-facing pair of the collar portions of the pole cores,there is formed an inter-collar portion gap, the stator further includesa plurality of magnets each of which is arranged in both a correspondingone of the inter-collar portion gaps and a corresponding one of theinter-coil gaps, and each of the magnets is configured to generatemagnetic flux in an opposite direction to magnetic flux that isgenerated by the corresponding unit coils and flows between thecorresponding pair of the collar portions of the pole cores through thecorresponding inter-collar portion gap formed therebetween.
 8. The motoras set forth in claim 1, wherein for each of the unit coils, theconnecting portion of the unit coil has its thickness set to be lessthan the thickness of other portions of the unit coil in the overlappingdirection.
 9. The motor as set forth in claim 8, wherein for each of theunit coils, the connecting portion of the unit coil has its thicknessset to be substantially ½ of the thickness of the other portions of theunit coil in the overlapping direction and its width set to besubstantially twice the width of the other portions of the unit coil inthe stacking direction.
 10. The motor as set forth in claim 1, whereineach of the first and second sub-coils of the unit coils has aquadrangular cross section.
 11. The motor as set forth in claim 10,wherein each of the first and second sub-coils of the unit coils has asubstantially rectangular cross section and is spirally wound withlonger sides of the rectangular cross section parallel to theoverlapping direction and shorter sides of the rectangular cross sectionparallel to the stacking direction.
 12. The motor as set forth in claim1, wherein the motor is of a radial gap type such that the stator isopposed to the rotor with a radial gap formed therebetween, and each ofthe first and second sub-coils of the unit coils has a substantiallytrapezoidal cross section which has a greater width on a radially outerside than on a radially inner side.
 13. The motor as set forth in claim1, wherein the motor is a DC motor where the stator constitutes anelectromagnet-type field and the rotor constitutes an armature, for eachof the unit coils, an outermost coil side of the first sub-coil of theunit coil, which is connected with a winding finish end of the firstsub-coil, constitutes a first lead portion of the unit coil, and anoutermost coil side of the second sub-coil of the unit coil, which isconnected with a winding finish end of the second sub-coil, constitutesa second lead portion of the unit coil, and each of the first and secondlead portions of the unit coils is arranged to adjoin, either in theoverlapping direction or in the stacking direction, another one of thefirst and second lead portions of the unit coils, and each adjoiningpair of the first and second lead portions of the unit coils areelectrically connected with each other.
 14. The motor as set forth inclaim 1, wherein the motor is used in a starter for starting an internalcombustion engine.